The metabolism of human procollagen is being studied with respect to: 1) the chemical structure of the nonhelical carboxyterminal sequences of type I procollagen; and 2) the number and modes of action of enzymes responsible for the conversion of procollagen to collagen. A radioimmunoassay for procollagen is being applied to sera and other body fluids of patients with diseases affecting connective tissues. Evidence is being sought for the specific binding of collagenous molecules to cell surface receptors.